The present invention generally relates to processes for producing perforated articles, and more particularly to a process for forming numerous holes in a curved panel suitable for use in an acoustic panel, such as of the type used in nacelles of gas turbine engines.
A typical construction used in aircraft engine nacelle components (for example, the engine inlet, thrust reversers, core cowl, and transcowl) and other aerostructures such as acoustic panels is a sandwich-type layered structure comprising a core material between thinner top and bottom sheets or skins. The core material is typically a lightweight material, often a foam or honeycomb metallic or composite material. A variety of metallic and composite materials can also be used for the skins, with common materials including aluminum alloys.
Nacelle fan duct flow surfaces typically include acoustic panels to suppress noise. A common form of acoustic panel comprises a contoured sheet/skin (sometimes referred to as a face sheet or acoustic skin) that faces the duct airflow, a backing sheet/skin, and an open-cell foam or honeycomb core therebetween. The acoustic skin is acoustically treated by forming numerous small through-holes that help to suppress noise by channeling pressure waves associated with sound into the open cells within the core, where the energy of the waves is dissipated through friction (conversion to heat), pressure losses, and cancellation by reflection of the waves from the backing skin. For some gas turbine engine applications, perforations on the order of about 0.03 to about 0.06 inch (about 0.75 to about 1.5 mm) in diameter and hole-to-hole spacings of about 0.06 to about 0.12 inch (about 1.5 to about 3 mm) are typical, resulting in acoustic hole patterns containing seventy-five holes or more per square inch (about twelve holes or more per square centimeter) of treated surface. Given the large number of holes necessary to acoustically treat airflow surfaces of acoustic panels, rapid and economical methods for producing the holes are desirable.
A process currently employed to produce acoustic skins is to perforate a flat aluminum sheet stock, such as by punching, to have the desired acoustic hole pattern, after which the sheet stock is formed to produce the arcuate shape required for the nacelle. Multiple heat treatments and forming steps are typically performed to reduce the likelihood of tearing the sheet stock during forming. During forming, the holes tend to elongate, often in a nonuniform manner such that the holes do not consistently have the same cross-sectional shape. While likely acceptable and adequate for many applications, an acoustic skin with nonuniform holes or a nonuniform hole pattern is likely to have unpredictable sound absorption performance that does not meet design requirements for more demanding applications. Other problems arise as a result of the holes often being punched in large sheet stock, which must be of sufficient size for the intended nacelle. The holes are typically punched over the full surface of the sheet, except for a small border along the edges of the sheet stock. As a result of the forming operation, holes may be present where none are desired for structural reasons. Though this problem can be solved by the use of doublers and reinforcements to maintain structural integrity, the solution comes with weight, part count, and cost penalties.
Another common process for producing acoustic hole patterns is to mechanically drill the holes in the surface of the acoustic skin after it has been formed. While this method overcomes the problems associated with fabricating acoustic skins from pre-perforated sheets, it requires the use of special tooling and machinery to place the holes in the proper orientation on the contoured non-planar skin. Though special-purpose machines designed to drill specific parts may be capable of as many as twenty-five holes per second, state-of-the-art mechanical drilling machines are typically limited to drilling about four holes per second. In addition to speed limitations, mechanical drilling processes tend to be expensive due to the special tooling and machinery required.
A more recent method for producing acoustic hole patterns is to employ an electron beam drilling technique, as reported in U.S. Pat. No. 6,358,590 to Blair et al. Face sheets (acoustic skins) with holes having diameters and spacings of up to 0.020 inch and 0.11 inch, respectively, are disclosed. Furthermore, Blair et al. teach that a decreasing hole diameter (in the direction toward the skin surface) is necessary. It appears unclear as to whether Blair et al. drill the face sheet before or after forming. Blair et al. also do not describe the fixturing for the face sheet during drilling, other than that a backing sheet is used.